Fuel cells have been extensively studied and developed to prevent global warming and solve energy problems in recent years. Hydrogen has attracted attention as a fuel for a fuel cell, and the development of a hydrogen application technique, such as its storage and a supply device, is also an important issue in spreading the applicability of fuel cells.
A polymer electrolyte fuel cell using hydrogen as a fuel, which can be mounted as a power supply for electric instruments, such as a notebook type computer, is disclosed by Japanese Patent Laid-Open Application No. H09-213359.
For a portable fuel cell, a hydrogen tank filled with a hydrogen storage alloy has been widely used to store hydrogen as the fuel.
Detection of the remaining amount of the fuel is a very important issue when a fuel cell is used. When liquid fuel, such as methanol, is used as the fuel, a part of the fuel tank may be transparent to visually check the remaining amount of the fuel. When hydrogen is used as the fuel and a hydrogen tank is filled with hydrogen, however, the remaining amount of the fuel cannot be visually checked.
In a fuel cell using hydrogen as the fuel, Japanese Patent Laid-Open Application No. 2002-252014 discloses a method of detecting a decrease in the remaining amount of the fuel by detecting a reduction in the output voltage of the fuel cell.
Furthermore, Japanese Patent Application Laid-Open No. 2001-295996 discloses a hydrogen gas supply device in which hydrogen is stored in a plurality of hydrogen storage tanks containing a hydrogen storage alloy. The pressure of one hydrogen storage tank is detected to sequentially switch from one hydrogen storage tank to another at the time this tank stops supplying hydrogen. A method of determining the remaining amount of the fuel based upon the number of hydrogen storage tanks that have stopped supplying hydrogen is also disclosed.
As a method of measuring the remaining amount of a fuel without a visual check in the case of using a liquid fuel is used, Japanese Patent Laid-Open Application No. 2005-98838 discloses a device for measuring the remaining amount of the fuel in which a plurality of liquid chambers are provided in a fuel tank, and the remaining amount of the fuel in each liquid chamber is detected to measure the remaining amount independent of the attitude of the fuel tank.
Hydrogen stored in a hydrogen storage alloy is generally released at a nearly constant release pressure until the remaining amount of hydrogen in the hydrogen storage alloy reaches a predetermined value. When the remaining amount of hydrogen is reduced to the predetermined value or less, the release pressure suddenly drops to stop the release of hydrogen. When the hydrogen fuel of a fuel cell is stored in a hydrogen fuel tank using a hydrogen storage alloy, only a very small amount of hydrogen is released from the hydrogen fuel tank after the output voltage of the fuel cell has been decreased due to a decrease in hydrogen release pressure from the hydrogen fuel tank.
When the remaining amount of hydrogen is detected by the method described in Japanese Patent Application Laid-Open No. 2002-252014, immediately before the fuel is exhausted, firstly the remaining amount of hydrogen is merely recognized to be small. Thus, the method cannot detect a predetermined remaining amount (for example, 50%, 10% and the like of the remaining amount of the total hydrogen storage amount), so that it is insufficient as a method of detecting the remaining amount in the fuel cell.
The method disclosed by Japanese Patent Application Laid-Open No. 2001-295996 requires that an on/off valve for supplying hydrogen be attached to each of a plurality of hydrogen storage tanks. As soon as one hydrogen storage tank finishes supplying hydrogen, the on/off valve is promptly manipulated to switch to another hydrogen storage tank to supply hydrogen. This causes a problem in that a device is enlarged on account of attaching the on/off valves and of automatically manipulating the on/off valves, thereby increasing the production cost of the device. The hydrogen storage tank for supplying hydrogen is switched at the same time when the hydrogen release pressure from the hydrogen fuel tank is decreased. However, a hydrogen supply amount (i.e., hydrogen release pressure) may momentarily drop when the hydrogen storage tank is switched. This produces a problem in that the use of the hydrogen supply device in combination with other devices, such as a fuel cell, makes the hydrogen supply amount unstable.
Japanese Patent Application Laid-Open No. 2005-98838 discloses a method of detecting the remaining amount of a fuel when a liquid fuel is used as the fuel. This method does not at all take into consideration the detection of the remaining amount of hydrogen in a hydrogen storage alloy. For this reason, such a method cannot be used for the detection of the remaining amount in the hydrogen storage tank using a hydrogen storage alloy.